The invention relates generally to electronically programmable semiconductor fuses, and more particularly to an apparatus and method of programming an electronically programmable semiconductor fuse.
Programmable semiconductor fuse devices are known in the art. For example, with reference to FIGS. 1-5, and initially to FIGS. 1 and 2, U.S. Patent Application Publication Nos. 2006/0087001 (Kothandaraman et al., the “'001 reference”) and 2006/0108662 (Kothandaraman et al., the “'662 reference”), both of which are assigned to the same assignee as the present application, disclose an electronically programmable semiconductor fuse assembly (or “eFuse”) 10 including a first conductive area 12 and a second conductive area 14 coupled by a fuse link 16. The '001 reference and the '662 reference are incorporated herein by reference in their entirety. The first and second conductive areas 12 and 14, as well as the fuse link 16 are formed from a polysilicon layer 24 and a metallic silicide layer 26 deposited over an insulating layer 22. As discussed in the '001 reference, the polysilicon layer 24 preferably includes a dopant. The insulating layer 22 may be formed, for example, from silicon oxide. The insulating, polysilicon, and silicide layers 22, 24, and 26, respectively, are formed on a semiconductor substrate 20. A capping barrier layer (not shown) formed, for example, from silicon nitride, may be provided over the insulating, polysilicon, and metallic silicide layers 22, 24, 26, respectively. The first and second conductive areas 12 and 14 are provided with contacts 18. The contacts are preferably formed from a metal such as tungsten.
An eFuse 10 programmed by an electromigration process changes from having a first resistance in an unprogrammed state to a second resistance, significantly higher than the first resistance, in a programmed state. To program the eFuse 10, a potential is applied across the fuse link 16 generating a programming current and raising the temperature of the fuse link 16. The electromigration process is affected by both the resultant current density within the fuse link 16, as well as by the temperature generated as a result of Joule heating generated by the current flow within the fuse link 16. With application of sufficient programming current, electromigration of metal within the silicide layer 26 occurs, with migration of the metal toward the anodic conductive area. Also, the dopant in the polysilicon layer 24 migrates toward the anodic conductive area. With migration of metal in the silicide layer 26 and of dopant in the polysilicon layer 24, the resistance of the fuse link 16 increases.
Programming an eFuse 10 requires providing a programming current of sufficient magnitude to reliably cause the desired degree of electromigration within the fuse link 16. However, exceeding the desired level of programming current can lead to excessive fuse link temperatures TFL. Specifically, the fuse link 16 has a rupture temperature TR at which the fuse link 16 is physically ruptured. Such rupture (uncontrolled explosion) of the fuse link 16 is undesirable as it can damage both the fuse link 16 as well as surrounding portions of the semiconductor device, rendering the eFuse 10 unsuitable for use. There is thus a relatively narrow range within which the programming current is both sufficiently large to cause an effective level of electromigration and sufficiently small to avoid heating the fuse link 16 beyond the rupture temperature TR.
The artisan will appreciate that variations inherent in the semiconductor manufacturing process can affect the range of acceptable programming current. For example, variations in the geometry or material composition of the fuse link 16 can decrease the range of acceptable programming current.
With reference now to FIG. 3, it is known to control the programming process of the programmable fuse 10 using a prior art current supply 40 comprising a programming field effect transistor (FET) 30 operatively coupled to control circuitry 32. The control circuitry 32 may include, for example, a pulse generator, one or more logic gates, or other conventional electrical components. The control circuitry is used to generate a pulse of voltage Vgs delivered to the gate of the programming FET 30. The eFuse 10 designer selects set points for programming FET gate voltage Vgs and programming voltage VFS corresponding to a programming current within the desired range of programming currents. For example, it is known in the art to generate a voltage pulse Vgs, typically having a magnitude in the range of 1.5 to 3.3 V, for a duration typically in the range of 5 to 250 microseconds, while simultaneously applying a programming voltage VFS, typically in the range of 1.0 to 3.5 V. With reference to FIG. 4, in one example of a prior art current supply 40, assuming application of a programming FET gate voltage Vgs of 2.0 V, in combination with a programming voltage VFS of 2.0 V, a programming current of roughly 15 mA is generated.
With continued reference to FIG. 4, it is noted that operation of the programming FET 30 in the transistor's saturation region, rather than in the linear region, is desirable, as in the saturation region, the programming current is relatively stable and insensitive to variations in the programming voltage. In the linear region, the programming current is substantially more sensitive to variation in the programming voltage. As discussed above, given that it is necessary to control the programming current within a specific range, additional variability in the programming current resulting from operating in the linear region is undesirable.
With reference to FIG. 5, the set points chosen for Vgs and VFS result in a theoretically satisfactory programming current (that is, a programming current sufficient to generate the desired degree of electromigration, without inducing a temperature in the fuse link 16 which exceeds the rupture temperature TR). However, given variability in the characteristics of the eFuse 10 device (including both the current supply 40 and the fuse link 16 itself), it is difficult to obtain a one hundred percent yield in the programming process. That is, some eFuse 10 devices programmed in the conventional manner will have either incomplete electromigration or will rupture due to excessive temperature.
A need exists, therefore, for an apparatus and method of programming an electronically programmable fuse which allows the eFuse 10 to be reliably programmed while also avoiding application of excessive programming current and the consequent potential for exceeding the rupture temperature of the fuse link 16.